Cellular antenna structures are used by cellular communications networks and service providers to mount antenna systems at a desired height from the ground for uninterrupted transmission and reception of cellular radio signals between the antenna system and a mobile device operated by a user.
A typical antenna base station structure is shown in FIG. 1 and comprises a tower or pole 1 and one or more axially spaced antenna support brackets 2 attached to the tower or pole 1. Attached to the brackets 2 and parallel to the pole 1 there is provided an antenna support 3, which is elongate and tubular. A hinged tilt bracket 4 and a joint 5 are attached to the antenna support 3 in axially spaced positions and an antenna 6 attached thereto. The length of the pole therefore determines the height of the antenna 6 from the ground. RF cables 7 are attached to the antenna 6.
Usually, the tower is firstly assembled at the location of installation and then the support brackets are mounted to the tower. The antennas are attached to the support brackets by means of mounting bolts and screws or other securing means, and are manually adjustable in three dimensions (heading (also known as azimuth), tilt and roll).
In order to facilitate this adjustment, the tilt bracket 4 is articulated to allow the top of the antenna 6 to move away from the support 3, and thereby rotate the antenna about the joint 5 (which has a horizontal axis of articulation) to adjust the antenna down-tilt angle. In the event that up-tilt is required then the tilt bracket 4 and the joint 5 can be swapped. The antenna heading (rotation about a vertical axis) can be adjusted by loosening the attachment of the bracket 4 and joint 5 to the support 3 and manually rotating the antenna 6 about the support 3. The antenna roll angle is usually set at zero and typically not adjustable.
The base station shown in FIG. 1 has a single sector (i.e. a single angular region covered by the direction antenna 6), but usually three regions (i.e. three antennas) are provided, each covering 120 degree arcs.
A typical cellular communications antenna is directional, comprising an elongate, planar metal reflector and a series of dipoles positioned in a line along the surface of the reflector. Usually, a cover is used to cover the dipoles in order to environmentally protect them. The cover is configured to be as transparent to electromagnetic radiation as possible in the cellular communications frequency range in order not to affect the antenna radio propagation characteristics. Cover materials having a substantial plastics material component such as GRP or ASA are commonly used.
As cellular networks are deploying broadband technologies for higher data rates (e.g. 3G/4G) the positioning of cellular antennas in a global sense is increasingly important. This is particularly true since broadband technologies are interference limited technologies. This means that higher the signal to interference ratio (C/I), the lower are the maximum data rates that can be achieved. The finite transmitted power from an antenna needs to be accurately directed to the planned target area in order to keep the signal to interference ratio under control. Accurate positioning of antennas reduces unwanted interference between adjacent sectors while directing the maximum signal power where is needed thus achieving minimum signal to interference ratio both inside and outside the target sector.
Capacity oriented network architectures should deploy antennas that can be dynamically adjustable such that their radiation pattern can redirect the finite network bandwidth from one area to another. More advanced antennas are remotely adjustable via electrical motors (or other means) such that their azimuth and tilt angles can be adjusted in order to provide the best possible coverage. For example, if a large number of users are in a certain area then a group of antennas can be realigned such that their respective coverage offers the required capacity for that area. As such, it is very important that the absolute direction of the antenna is known, so that its position can be accurately adjusted.
Desired antenna position is usually determined through a radio planning process, carried out by the network operator. This process provides details of the desired global position of the antenna, as well as specific values of heading, tilt and roll.
To achieve high network performance, provide high quality radio link transmissions and reception and ensure high spectrum efficiency, directional antennas must be aligned with minimum inaccuracy (less than ±1°) in the degrees of freedom (heading, tilt and roll). Accurate alignment of directional antennas is of paramount importance in a competitive wireless communication industry, as even small errors in azimuth and tilt alignment (more than ±50 for azimuth and more than ±10 for tilt) can seriously degrade radio network quality.
Several prior art solutions are currently available for antenna alignment purposes. For example, US20090021447 and US20110225804 each describe devices for measuring the orientation of an antenna in three degrees of freedom, i.e. heading, tilt and roll. The devices are directly secured to an antenna by a technician, rigger or climber and display the measurements performed in real time allowing the user to accurately align an antenna to the desired directions.
One deficiency of these prior art devices is that they need to be operated by a technician, rigger or climber at the location of the antenna. As a result, the use of the measurement devices described in US20090021447 and US20110225804 always need to be operated by a user in real time. Every time the antenna needs to be adjusted, the technician, rigger or climber needs to scale the tower, mount the system to the antenna and carry out the required adjustment.
Such devices are also very expensive (many thousands of Euros), and as such cannot be permanently mounted on antennas as this would significantly increase unit cost, and therefore the capital expenditure (CAPEX) of the operator.
Measurement uncertainty is the sum of systematic and random uncertainties introduced in the measurement process. Systematic uncertainties are introduced from a measurement device while random uncertainties are introduced from the method followed by operating the measurement device and collecting and interpreting the measurement results (i.e. human error in use of the device). The measurement devices described in US20090021447 and US20110225804 cannot exclude the human error introduced in the measurement process because they need to be operated at the antenna location by a technician, rigger or climber. They also need to be affixed to the antenna in the correct manner each time adjustment is required. This is generally undesirable.
Furthermore, due to the modern networks' dynamic nature, repeatable antenna azimuth and tilt re-adjustment during the lifecycle of a base station site (for one or more antenna systems) is required; therefore, the antenna brackets, the antennas or the antenna structure itself should be capable of facilitating such needs. Antenna azimuth and tilt readjustment has to be performed with the same high degree of accuracy as the original installation.
Antenna azimuth and tilt re-adjustment should ideally take place without the need to climb on the tower top and manually adjust the antenna position. Manual reposition involves high operational expenditure (OPEX) due to climbing, as well as health and safety risks for antenna technicians, riggers and climbers. It is also desirable to reduce human exposure to the strong electromagnetic fields proximate the antennas. At present, most network operators inhibit antenna operation during the time that such works are performed on the antenna system, thus preventing coverage from the selected antenna and/or base station. This is also undesirable.
As discussed, the devices disclosed in US20090021447 and US20110225804 do not disclose remote (only local) re-adjustment of the antenna orientation. However, should remote re-adjustment be used, this requirement cannot be satisfied by this prior art.
A prior art antenna that offers built-in remote azimuth and tilt adjustment by electromechanical actuation and provide heading, tilt and roll measurement means is disclosed on US20090195467. A problem with this prior art is that the adjustment mechanism is integrated with the antenna itself. This forces the operator to adopt one type of antenna, and also forces them to invest in a new antenna. Preferably, the operator should be able to select the appropriate antenna for its radio characteristics in the first instance.
A further problem encountered by modern cellular antennas is in the use of the MIMO (multiple input multiple output) protocol. This type of smart antenna technology currently uses arrays of cross polarized dipoles to form multiple antennas under the same housing (radome) in order to increase data bandwidth performance due to better exploitation of both transmit and receive de-correlated RF paths (polarization diversity transmission and reception). Such antennas, although offering high spectrum efficiency, are difficult to install on the antenna structure due to their physical dimensions, complexity of installation and aesthetics. Furthermore, antennas supporting MIMO technology for various configurations (i.e. 2×2, 4×4, etc.) are significantly more expensive than the legacy cross polarized antennas used today.
Another disadvantage of the prior art MIMO antenna technology is that due to their size, steering capability of the antenna radiation pattern cannot be easily achieved with electromechanical actuation. This results in an inherent limitation of the antenna technology as it cannot satisfy the modern networks' dynamic needs, where repeatable antenna azimuth and tilt re-adjustment during the lifecycle of a base station site (for one or more antenna systems) is required.
Alternative use of the MIMO technique deployment requires the de-correlation to be achieved by spacing dipoles apart (horizontally or vertically, by a specified distance in wavelengths (λ)—distance depends on both MIMO performance requirements as well as other parameters such as RAN technology used and modulation/coding schemes) in order to achieve increased data bandwidth performance due to better exploitation of both transmit and receive de-correlated RF paths (space diversity transmission and reception). In order to manage best possible de correlation effects by spacing, two or more antennas need to be spaced apart in such a way that two antennas should point in exactly the same 3-dimensional direction having known distance in wavelengths (λ) to each other. This is required for the de-correlation to take maximum effect per performance targets and technology deployed
MIMO technique deployment by spacing dipoles (or antennas) apart with high precision mapping in three degrees of freedom (heading, tilt and roll) for two antennas at a distance apart can be challenging for antenna technicians, riggers or climbers (for the reasons discussed above). Furthermore, accurate fixation of two antennas at a specified horizontal or vertical distance in wavelengths (λ) whilst also achieving precise parallelism and/or verticality to each other is also challenging. The aforementioned installation problems, when attempting high precision positioning and alignment of two or more antennas for MIMO technique deployment by spacing, is generally difficult (if not impossible) to handle with today's installation practice and tools.
A further problem with directing antennas in the desired direction, in particular by remote actuation, is “play”, or free movement, in the actuation system. The use of electric motors and gear trains results in some inevitable backlash which can cause the antenna to move in use. In particular worm gears (which offer an advantage in gearing) have typically high backlash.
A problem with existing antenna installations is the fact that they are generally exposed to the external environment, i.e., repeated cyclical wind loading on the antenna. The repeated buffeting of the antenna over time may cause wear in the antenna mounting components, in particular if a remotely driven antenna is provided. Therefore the life cycle of these components is limited. One solution is to cover the entire assembly with a radome, however this restricts the space available requiring any adjustment mechanism to be integrated with the antenna itself.
GB2251521A discloses an orientation adjusting device for an antenna which uses a worm gear. A problem with such arrangements is that backlash can be a problem, and complex mechanical modifications to the drive train (such as that disclosed in the document) are required to alleviate backlash, adding cost and complexity to the assembly. Also, wind loading on the antenna acts to repeatedly and/or continuously back drive the drive mechanism imposing the risk of failure over time. Although worm gears cannot be back driven, the gears have to be over engineered to cope with the induced stresses from e.g. wind loading, and in the case of backlash the potential repeated, small movements (which may cause fatigue). In the case of a gearbox, this means that the gears become significantly larger and heavier than they would otherwise need to be.
A still further problem with remote actuation of antennas is that the electrical specifications defined by industry standards set maximum currents for the operations to be performed on the antenna and associated devices. This limits the size of the motor to be used for high torque applications, and necessitates a gearbox so that a smaller, lower current motor can be used, which in turn introduces further cost and complexity into the system. As such, in a case that a remote actuation system needs to be compliant to the industry standards, the proper balance between the motor and the gearbox size need to be accounted in order to satisfy the application.
DE9010416U1 discloses an antenna mounting apparatus which is configured to adjust the tilt angle of the antenna using a number of holes at the top bracket to manually secure the antenna in position.
EP1753075A1 discloses an antenna mast in which the azimuth of each antenna can be altered by manual rotation about a pivot point. The antenna may be secured in position by aligning a hole on the antenna bracket with one of several holes on the mast, and the user passing a bolt (secured by a nut) through the aligned holes to secure the antenna. A problem with this invention is that positive user intervention is required to secure the antenna in place. As such, user error can result in an unsecured antenna.
WO 00/46872 also uses a bolt with an array of holes to manually position and lock the azimuth of the antenna.